yocto/actuation,doc: Update Actuation Demo to v2.1

Update the Safety Island Actuation Demo to the latest version
v2.1, mention the change in the release note.

After this change, some variable and values of rst_prolog in
conf.py won't be used anymore, so remove them.

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>